I found the solution
the problem was in audio file extension , mp3 files for some reason are not working when I converted the mp3 file to wav extension it worked flawlessly
also wav keyword must be in the buildozer init

NoneType means that your function call failed or it returned an unexpected result.

In your case,

self.sound expects to load the mp3 file and when it is called by self.sound.play(), the file was either not fetched or it returned an unexpected result

import kivy
from kivy.app import App
from kivy.uix.button import Label
from kivy.core.audio import SoundLoader

class HelloApp(App):
    def build(self):
        self.sound = SoundLoader.load('back.mp3') # open the background music
        self.sound.play() # play the sound
        return Label(text='>>>>>')
if __name__=="__main__":
    HelloApp().run()

this is my code it works fine on linux
but when i try to make apk by buildozer
the app crashes upon start
and the logcat command gives this

[WARNING] [Audio ] Unable to find a loader for <back.mp3>
: Traceback (most recent call last):
: File “/home/moh/audio2/.buildozer/android/app/main.py”, line 15, in
: File “/home/moh/audio2/.buildozer/android/platform/build-armeabi-v7a/build/python-installs/myapp/kivy/app.py”, line 829, in run
File “/home/moh/audio2/.buildozer/android/app/main.py”, line 9, in build
AttributeError: ‘NoneType’ object has no attribute ‘play’
Python for android ended.